1. Field of the Invention
This invention relates to a method for consolidating subterranean formations using diluted resin and overflush oils to extract the resin diluent. In one aspect, it relates to the use of an improved diluent for resins used in such operations.
2. Description of the Prior Art
The use of resins as a sand control means in oil wells, gas wells, water wells, and similar boreholes has long been employed. The plastic consolidation treatments, as they are commonly called, involve the injection of a liquid resin into the formation and thereafter causing the resin to cure to an infusible state. The resin cements the sand grains together forming a permeable consolidated sheath around the wellbore. In order to be successful, the consolidation treatment must not only provide a high strength in the consolidated interval, but must retain a high percentage of formation permeability through which the produced fluid can flow.
In certain types of treatments, adequate permeability can be achieved only by the use of an overflush liquid which functions to remove resin from the pore space.
Opposing mechanisms are involved in the overflushing process: resin must be removed to provide permeability but sufficient resin must be left on the sand grains to provide adequate consolidation strength. An improper balance of these two conditions will result in the failure of the consolidation treatment.
The problem of achieving the proper balance of permeability and strength in the treated interval when an oil overflush is used is even more difficult in resins which employ diluents. These resins, exemplified by expoxy resins, are available only as viscous liquids or solids. In order to handle and inject these resins into the formation, they must be diluted with a suitable diluent. Moreover, since the sequential injection of the treating fluids normally exhibits a progressively increasing viscosity (in the order of injection) to provide favorable mobility ratios, high resin viscosities will result in even higher viscosities of the following liquids and, consequently, still more difficulty injecting the required overflush oil.
With diluted resins, the overflushing liquid must selectively extract the diluent from the resin. If substantial amounts of the diluent are left in the resin, the resin, upon curing, will not attain sufficient compressive strength for the purposes of sand consolidation treatments. Thus, merely displacing the diluted resin from pore space to provide permeability is wholly unsatisfactory.
It might be thought that an effective single component resin diluent could be thoroughly extracted from the resin by simply overflushing with a larger quantity of overflush oil. However, this practice has shortcomings. Laboratory tests have shown that the use of large amounts of the overflush oil will adversely affect compressive strength of the consolidated interval. Large amounts of the soluble diluent render the resin slightly soluble in the overflush liquid. Thus, large amounts of overflush oil will, either by erosion or by dissolution, remove substantial amounts of resin from the sand grains, with the result that there is insufficient resin left on the sand grains to provide a high strength consolidated interval.
In summary, the resin diluent must simultaneously be effective in reducing resin viscosity and be easily extractable by the overflush liquid. Unfortunately, resin diluents that have been used in the past either exhibit good viscosity reduction properties or good extractability by the overflush liquid, but not both of these important properties.
An example of one common diluent used with epoxy resins is ethyl acetate (See "Consolidation of Silty Sands with an Epoxy Resin Overflush Process" by E. H. Richardson et al., published in Journal of Petroleum Technology, September, 1970.) This diluent is readily extractable by the overflush oil but it is only moderately effective in reducing the viscosity of a liquid epoxy resin. Relatively large amounts of ethyl acetate are required to reduce the resin viscosity sufficiently to permit easy injection into the formation. High dilutions result in lower resin retention and, therefore, lower strength.
Another diluent commonly used with epoxy resins is acetone (see U.S. Pat. No. 3,612,181). Acetone is more effective in reducing resin viscosity than is ethyl acetate, but unfortunately it is not easily extracted by the overflush oil. In order to permit removal of substantial amounts of acetone by the extraction process, its concentration is normally limited to about fifteen weight percent in the resin solution. This reduces the resin viscosity to only about 90 centipoises, which, as noted above, is unsuitable for many treatments.
Reactive diluents such as propylene oxide, styrene oxide, octylene oxide, and the like have also been proposed for use with epoxy resins (see U.S. Pat. No. 3,176,768). Except for propylene oxide, most of the reactive diluents are not particularly effective in reducing resin viscosity. Propylene oxide, while an effective viscosity reducer, is extremely hazardous to use because it is known to react spontaneously with many common chemicals such as copper or brass to form explosive acetylides.
Plastic sand consolidation treatments which rely on the use of dilute resins and diluent extractants have been widely used over the years and have proven satisfactory as a sand control technique. However, it is believed that they have not realized their full potential in developing long life and highly permeable consolidations. It is believed that one of the major problems with these systems is due to the type of diluents commonly employed.